Maintaining Water and Nitrogen Balance (Homeostasis Principles) — AQA GCSE Biology
Homeostasis keeps the body's internal conditions stable. This topic introduces the principles and the need to balance water and nitrogen.
What is homeostasis?
Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function, in response to internal and external changes. It is important for keeping enzymes working and cells functioning properly.
Conditions controlled in the human body include:
- blood glucose concentration,
- body temperature, and
- water levels.
Components of control systems
All automatic control systems include:
- Receptors — cells that detect a change (stimulus).
- Coordination centres — such as the brain, spinal cord and pancreas, which receive and process information.
- Effectors — muscles or glands that bring about a response to restore the optimum level.
Many systems use negative feedback, where a change triggers a response that reverses it.
Water and nitrogen balance
The body must balance water gained (drink, food, respiration) with water lost (urine, sweat, breath). Excess amino acids cannot be stored, so they are broken down in the liver in a process called deamination, forming ammonia (toxic), which is converted to urea and removed by the kidneys in urine.
Exam tips
- Define homeostasis and explain why it matters (enzymes and cell function).
- Learn the three control-system parts: receptor → coordination centre → effector.
- Explain negative feedback in your own words.
- Excess amino acids → deamination in the liver → urea, excreted by the kidneys.